In the systems and controls theory as well as in the practice it is of interest to follow input commands as closely as possible and to minimize effects of external (load) disturbances, in both transient and steady state. Further it is of ultimate interest to develop a generalized algorithm for the synthesis of control systems of zero-order/instantaneous response and infinite disturbance rejection ratio.
Theoretically, both zero-order and infinite disturbance rejection ratio may be achieved using a negative feedback control theory and employing an infinite gain, topologically located in a loop before the point of entry of disturbance, but, in such a case, the system will necessarily become unstable, so that, with this classical approach, no solution can be reached. This remains a classical problem in system and control theory and practice.
In modern control theory the problem basically remains the same. See, for example, the attempts and the results as well as the discussions in G. H. Hostetter et al, "Design of Feedback Control Systems", Holt, Rinehart and Winston, 1982, Section 7.9 (A Magnetic Levitation System), pp. 423-430, B. C. Kuo, "Automatic Control Systems", Prentice-Hall Inc., 4th Ed., 1982, Section 8.8 (State Feedback With Integral Control), pp. 529-536, W. A. Wolovich, "Robotics: Basic Analysis and Design", Holt, Rinehart and Winston 1987, Chapter 8, and in particular Section 8.4 (Inverse Dynamic Feedforward Control) and Section 8.5 (Nonlinear and Two-Part Control), pp. 311-345, and C. L. Phillips and R. D. Harbor, "Feedback Control Systems", Prentice-Hall Inc., 1988, Chapter 13 (Modern Control Design), and especially Section 13.1 (Pole-Placement Design) and Example 13.2 with the concluding remarks, pp. 509.gtoreq.518.
We will cite at this point a portion from the concluding remarks above (pp. 518): "It appears from the preceding example that we can choose the magnitude of the real part of the roots arbitrarily large, making the system response arbitrarily fast. For the system model, we can do this. However, as the time constant of the system becomes smaller, the gains increase." This conclusion coincides with the problem stated in connection with the classical control theory and practice.
An infinite disturbance rejection ratio, i.e., load independence, has been achieved employing a positive feedback as described in the U.S. Pat. No. 4,885,674 "Synthesis of Load-Independent Switch-Mode Power Converters" by these same two inventors Lj. Dj. Varga and N. A. Losic, and in the patent application No. 07/323,630, November 1988, "Synthesis of Load-Independent DC Drive System" by N. A. Losic and Lj. Dj. Varga, and patent application No. 07/316,664, February 1989, "Synthesis of Load-Independent AC Drive Systems" (allowed for issuance December 1989) by N. A. Losic and Lj. Dj. Varga, now U.S. Pat. No. 4,967,134. The inventions have been generalized with respect to providing infinite disturbance rejection ratio for both switch-mode power converters and electric motor drive systems, including step-motor drive systems, in a copending and coassigned application No. 07/452,000, December 1989, "Synthesis of Zero-Impedance Converter" by Lj. Dj. Varga and N. A. Losic.
Furthermore, a synthesis of electric motor drive systems of infinite disturbance rejection ratio and zero-order dynamics/instantaneous response is described in a copending and coassigned applications by N. A. Losic and Lj. Dj. Varga, "Synthesis of Improved Zero-Impedance Converter", December 1989, and "Synthesis of Drive Systems of Infinite Disturbance Rejection Ratio and Zero-Dynamics/Instantaneous Response", January 1990, in cases of not closing and closing additional velocity and position feedback loops, respectively.